Until relatively recently, cables of suspension bridges, bridle chord bridges and cable-stayed girder bridges were made of steel wire ropes (ropes, locked coil ropes, helical wire strands, locked wire strand cables) since the "aerial spun process" developed by J. Roebling was not used for bridges of very large span. Particularly in the United States, great efforts have been made during the last few years, because of disadvantages inherent in the aerial spun process, to use in some other fashion the advantages inherent in parallel steel wires of high tensile strength and endurance strength.
Prefabricated parallel wire strands were manufactured and used in the construction of a wide span suspension bridge for the first time in 1968/1969. Japanese companies developed this process further, using parallel wire strands for suspension bridges as well as in construction of cable-stayed bridges bridges in Japan.
In Europe and especially in Germany attempts were made to cope with the disadvantages connected with installation of locked wire strand cables by endeavoring to obtain, by experiment, further information concerning cable stretch, creep, working modulus, and modulus of elasticity and by using the thus ascertained approximate values in such calculations. One disadvantage of the last mentioned type of construction is that the load carrying capacity of the total cable is less, due to stranding, than the sum of the load carrying capacity of the individual wires and that the modulus of elasticity of the total cable is only about 80% of the modulus of elasticity of the individual wire. The pertinent specification of the Federal Republic of Germany, DIN Standard E 1073, therefore, stipulates for bridge constructions that the permissible stresses in the case of stranded wire rope are to be set relatively about 8% less than in the case of universal, parallel guided wire. In addition thereto, the locked wire strand cable has a certain cable stretch and the determination of the so called "working modulus" gives less exact results, so that precise mounting is rendered difficult. Furthermore, the creep values of the locked wire strand cable under continuous load are substantially greater than those of a cable consisting of parallel wires. If, in order to support a bridge girder against the action of traffic loads, it is desired to obtain with locked wire strand cables the same effective stiffness as with parallel wires, other physical properties of the steel wire being the same, more material is required due to the 20% lower modulus of elasticity.
These factors make it appear of advantage from a technical and economic standpoint to use in the future predominantly parallel wire strands instead of locked wire strand cables. This is all the more advantageous as it has been realized in the meantime that the cable-stayed bridge constructions have economic and technical advantages with respect to stiffness as well as lack of sensitivity to wind oscillation as compared with suspension bridges of up to main span lengths of 600 meters and even 800 meters.
In July 1972 in Germany a bridge was opened to traffic over the Rhine near Mannheim which had been constructed with use of parallel wire strands (295 wires of a diameter of 7 mm., steel 140/160). Technical advantages of this solution became clearly evident. On the basis of local conditions it was necessary, in order to afford protection against corrosion, to fill the spaces between the wires completely with a plastic material to which an activated pigment had been added. It was difficult, however, to transport the stiff wire strands. For this reason, they had to be fabricated on the site of construction. Therefore, this method is suitable only for special cases since it is rather expensive. In order to achieve economical manufacture, prefabrication of such parallel wire strands is therefore an almost necessary objective.
The Japanese steel industry succeeded in manufacturing shorter strands of 154 wires of 5 mm. in diameter in a centrally located plant, winding said strands, after providing them with anchoring heads, onto reels of a diameter of about 3 meters and transporting them to the building site in this form. The manufacture of parallel wire strands of 61 and 91 wires of a diameter of 5 mm. of great length at the factory is customary in Japan. The apparatus required for this purpose is stationary.
The problem which existed for many years in the United States of America and later on in Japan resides in the fact that strands with parallel wires are very difficult to wind on reels. If equalization in length is no longer possible after completion of the anchoring at the end of the strands, the wires of the outer region are stretched upon bending during winding and tensile stresses are imparted thereto. The wires of the inner region are compressed and must become shorter if the cross-sections are to remain conformal. Then it is not possible to activate within the wires which represent a cross-section of the strand, shearing stresses sufficient to compensate for the different and oppositely directed normal stresses. The cross-sections of the strand tend to fall apart during winding up, a process which is known in the United States as the "bird cage effect".
It was necessary in the United States and Japan to construct heavy machines capable of producing large guiding and clamping forces in order to control these phenomena by turning the strand back and forth during winding up in such a fashion that alternately the outer region of the wire faces the inside and then is turned back again in the opposite direction. The same process is experienced by the wires of the inner region as a result of which the compressive and tensile tendencies which are produced within the wires during the winding on the reel can be counteracted. The parallel wire strands which are surrounded by elastic bands thereby change their original cross-section only slightly. After the unwinding of the strand at the building site, the wires are again parallel and ready for the commencement of the assembly.
The manipulation described by which the tendencies produced in the wire are neutralized within the strand during the winding require very complicated and costly machines which develop high forces. Therefore, the danger exists that the wire material is subjected to uncontrolled excessive stresses by the backward and forward turning thereof. Development of such large machines is worthwhile only for the manufacture of parallel wire strands which are required for forming main cables of large suspension bridges.
Technically simpler and more economic processes for manufacturing prefabricated parallel wire strands are required for the spans which can be mastered in general with cable-stayed bridges cables in Europe and recently also elsewhere in the world, as they are by far the majority.
Retention of the original cross-section of the strand resulted in difficulties upon winding on reels due to differences in length occurring upon bending and which could be controlled only by applying large forces requiring corresponding expenditures. It is advisable, therefore, to yield to natural tendencies of the wires and to avoid in all cases compressing in stranded condition.
The invention described in my earlier application above referred to makes it possible to pre-manufacture in a factory and to bring to the construction site, in a state of being wound on drums, the tension members of great bearing capacity which are needed for spanning bridges with wide spans (suspension bridges and slanting-cable bridges of steel and reinforced concrete) as well as for other technical tasks in which high tension forces must be absorbed in a concentrated form.
By the processes proposed in such application it is for the first time possible to utilize fully the physical advantages which can be achieved by using strong strands with completely parallel wires of high strength and also enlarged diameter (7-9 mm) with respect to the rigidity of the bridge girder support, including the completely elastic behavior of the tension members in slanting-cable bridges and the elimination of the inhibiting weather influences which very often strongly impede the manufacture of the main bearing cable for suspension bridges in the outside-air spinning process. The progress achieved consists in that it has been made possible to wind parallel wire strands of such strength, with 199 wires and more, on drums of a diameter of 2.50 m. to 3.50 m., without the occurrence of any noteworthy stresses or inherent tensions in the wires during this process. Whereas only strands with 61 to 91 wires of 5 mm. diameter have been wound upon a drum in a manner developed in the United States and Great Britain, due to stresses resulting from the necessary twistings of the total strand, in my invention described in my earlier application the number of wires is not limited with regard to the winding process. Large-scale tests with wires of 7 mm. diameter and a rated strength of 170 kp/sq. mm. have already lead to good results with improvised devices and have proved the functioning of the novel process.
My earlier invention described in Application Ser. No. 385,683 solves the aforementioned problems in two different ways or methods. In the first method, the parallel wire strands are fanned out, and the wires, arranged in layers side by side on the drum, are bent almost completely about their own axis, which operation, at a bending radius of 2.50 to 3.50 m. and the small thickness of the object to be bent, is not difficult. By a device disposed adjacent to the previously mounted anchoring bodies at the beginning and end of the strand, the differences in the individual wire lengths which occur as a result of the initiated bending of the strand beginning or end, and of the thus produced change in the original diameter, are fully compensated or neutralized in such a manner that in this process any substantial stresses and therefore tensions cannot occur.
In the second method, the differences in length between the wires positioned on the outside and those on the inside in the parallel wire strand are, with tensions being avoided, compensated by using two drums that rotate in the same direction and at the same speed. The strand retains its shape in a flexible cover tube, and the excess lengths at one drum are neutralized by the deficiencies in length occurring at the other drum. The wires must shift with respect to each other within the strand synchronously with the winding process, for the purpose of length compensation, which fact requires, on the other hand, at the unwinding a motion in the opposite direction. The forces of friction of the wires against each other are decreased by suitable means or temporarily largely eliminated.